The present invention relates to a turbo compound engine capable of recovering the exhaust energy of an internal combustion engine, and more particularly to a turbo compound engine including a generator drivable by the exhaust energy of the engine and a motor operatively coupled to the output shaft of the engine and energizable by the output of the generator for recovering the exhaust energy.
There have been developed in recent years thermally insulated engines having various components made of ceramics, including an outer wall of an exhaust manifold, a cylinder liner, an insulating plate for a cylinder head, and a piston, for example. In such thermally insulated engines, it is not necessary to cool the engine by radiating the heat generated inside the engine, but the energy of a high-temperature exhaust gas discharged by the engine is recovered and fed back to the output shaft of the engine for increasing the output power from the engine.
One known method of recovering the exhaust energy of an engine utilizes a multistage gear speed reducer mechanism for reducing the speed of rotation of a turbine which is rotated by the exhaust gas to transmit the rotative power of the turbine to the crankshaft.
Japanese Laid-Open Patent Publication No. 59(1984)-141713 discloses an exhaust energy recovering device. According to the disclosed device, the exhaust turbine of an AC generator is rotated by the energy of an exhaust gas discharged from an internal combustion engine, and an induction motor operatively coupled to the output shaft of the engine is driven by the output from the AC generator to recover the exhaust energy. The device also includes means for determining the magnitude of a load on the engine to control the amount of intake air, so that the temperature of the exhaust gas can be increased when the engine load is small.
In the former method of recovering the exhaust energy, the transmission efficiency of a one-stage gear speed reducer is in the range of from 0.9 to 0.95, and hence, the transmission efficiency of a three-stage gear speed reducer is low, i.e., about 80%. The gear mechanism is complex and highly costly. Furthermore, there is a problem with respect to the location where the gear mechanism is to be placed, since the space in the engine compartment is small on a small-size passenger car. According to the latter exhaust energy recovering device, the amount of intake air is controlled dependent on the magnitude of the load on the internal combustion engine, and the AC generator is rotated by the energy of the produced exhaust gas to generate AC power, which is controlled to energize the induction motor for rotating the output shaft of the engine. However, this device is disadvantegeous in that the extent to which the output shaft is driven is controlled indirectly, and the motor cannot be driven at a rate commensurate with the rotation of the output shaft.